1. Field of the Invention
The present invention relates generally to an optical disk apparatus which reproduces data from a disk used as a recording medium. More particularly, this invention relates to data restoration that is executed when data contained in one or more data memory areas called a sector is damaged.
2. Description of the Related Art
Conventional optical disks, such as a laser disk, compact disk and CD-ROM are called a read-only disk. A read-only disk has a read-only memory area (hereinafter called "ROM area") where audio data, image data, code data, etc. can be stored. A writable disk is a so-called magneto-optical (MO) disk which has a writable memory area (hereinafter called "MO area"). MO area can store audio data, image data, code data, etc.
Another type of optical disk called a partial ROM utilizes a large memory capacity and has both a ROM area and an MO area on a single disk. Partial ROM disks provide an advantage when different computer operating systems (hereinafter OS's) are stored in the ROM area. Such disks ensure stable data exchange between different computer systems. There are many computer systems, which require a separate OS for each computer system used. Data exchange between computers having different OS's, generally speaking, can not occur without data conversion.
Each of the ROM and MO areas of the partial ROM has a plurality of tracks divided into data memory areas called sectors. Data stored in each sector in the ROM area can be read but cannot be overwritten. When data in the ROM area is damaged, however, it is desirable to be able to restore the data.
An optical disk apparatus, which reads or writes data from or to a partial ROM, restores data in the following manner when data in one sector, within a certain range in the ROM area, is damaged. When a read error occurs for data in one sector, the optical disk apparatus corrects the error in that sector using an error check and correction (ECC) code provided in the sector. If error correction with the ECC code cannot be accomplished completely, the optical disk apparatus restores data in the corrupt sector (hereinafter called "defective sector"). To do this the optical disk apparatus uses data in a parity block provided for each group of sectors located within a certain range (e.g., one track). Parity data, which is obtained through an exclusive OR operation of data in all the sectors in one group, is stored in the parity block. To restore data in a defective sector, the optical disk apparatus performs an exclusive OR operation on data in all the sectors in a sector group in question, excluding the defective sector, and stores the parity data in the parity block.
To restore data in a defective sector when error correction protocol via an ECC code has failed, however, this conventional optical disk apparatus cannot restore more than one piece of data in the defective sector. If there are two or more defective sectors in one sector group, the conventional optical disk apparatus can restore data in one of the defective sectors using the parity data but cannot restore data in the other defective sector. This limitation in error correction protocol reduces the error correction performance and impairs the general usage of the optical disk despite its advantage of having large memory capacity. This reduces the value of optical disks.